Romans 4:13 : For not through the law was the promise to Abraham or to his seed that he should be heir of the world, but through the righteousness of faith. The marginal reading has "through law." The Greek has no "the" before "law" in this verse. Abraham did not receive the promise through law, evidently meaning that the promise was not given him on account of his perfectly keeping any law. That he should be the heir of the world is not definitely stated in any of the promises made to Abraham. The promise here re-ferred to cannot be the land promise, for that promise did not include the world. And Paul's argument in the remainder of the chapter shows that he did not have the land promise in mind.
When God called Abraham out of Ur of the Chaldees, he promised to make of him a great nation, and then added the promise that refers to Christ: "And in thee shall all the families of the earth be blessed" (Genesis 12:1-3). That this promise includes all the nations is plainly declared when God renewed this promise to Abraham: "And in thy seed shall all the nations of the earth be bless-ed" (Genesis 22:18). When God made the covenant of circumcision with Abraham, he referred to the prom-ise made to Abraham in Ur of the Chaldees, saying "For the father of a multitude of nations have I made thee" (Genesis 17:5). God had already constituted him a father of a multitude of nations when circumcision was commanded. In a fleshly sense Abraham was not the father of a multitude of nations; in a spiritual sense he was. Jesus was made heir of all things.
Hebrews 1:1-2; Psalms 2:7-8 shows that it was the people to which Christ became heir.